Meteorology Basics For Smart Boating
Published: Wednesday, May 6, 2020
By: Melissa Walsh
I’ve been a Great Lakes boater my entire life and have plenty of wild storm stories to tell as a result.
During a boat trip home from Cedar Point in 1978, my dad piloted us across Lake Erie during what he recalls as the worst lake storm he’s been in. I, as my dad’s 11-year-old co-pilot and chart-reader, remember riding those giant waves being as thrilling as any roller coaster ride. Thankfully, my dad had the boating skills necessary to get us through that stormy darkness safely.
With this deep respect for the power of our lakes, I attended an all-day seminar on Great Lakes basic marine weather forecasting to gain a critical education from marine weather expert Mark Thornton on basic meteorology, interpreting weather graphics, wind and precipitation predictions, low-pressure and high-pressure systems, radar and satellite imagery, and more.
Thornton maintains LakeErieWX.com, a go-to weather forecasting site that each Great Lakes boater should bookmark in their web browser. He offers weather seminars and workshops in cities around the Great Lakes basin and publishes compelling articles about Great Lakes weather on the site.
The Basics: Check the Barometric Pressure
Thornton advises having a barometer on board—and a good one. Having a digital barometer is best and you should ensure it is calibrated regularly. Barometer manufacturers that Thornton recommends include Kestrel or Weems and Plath. He calls the hand-held Kestrel model “a mini weather station.” The key features of an ideal barometer include both the ability to view recent air-pressure history in a graph form as well as the ability to produce audible warnings of dropping pressure, keeping you alert to sudden changes.
At the dock, before heading for a day on the lake, a boater should check the recent barometric pressure trend, or “the weight of air molecules above your location pressing down on the surface of the Earth.” Since weather maps are adjusted to sea level pressure, it is important to calibrate a new barometer to your specific location. However, even an uncalibrated barometer offers value, Thornton says, because it will show trends in rising or falling pressure, which can help you understand active, current changing weather.
“There is no threshold between high pressure and low pressure,” says Thornton. “It’s always relative to the prevailing weather pattern.”
Though fluctuations in barometric pressure in the Great Lakes is typically +/- 5 percent (5%) during the boating season on any ‘normal’ day, you must remain vigilant since changes in pressure usually signal the approach of weather systems, and could significantly impact your decision-making. Falling pressure is a typical sign of inclement weather on the way, though not all falling pressure brings bad weather. The steepness of the decline in pressure provides valuable insight into the strength of the approaching system. Conversely, rising pressure indicates fair weather ahead.
“A great way to avoid trouble on the water is to watch the trends on your barometer,” Thornton advises.
Know How to Interpret a Weather Map
A boater’s reading of a weather map begins with identifying the lows (red Ls) and the highs (blue Hs) by using the “isobars.” Isobars are the lines showing the contours of constant sea-level barometric pressure (in millibars) and are usually plotted at 4mb intervals. Isobars also define additional features, such as “troughs” and “ridges.” Whether it’s a trough, which is always associated with lows, or a ridge, which is always associated with highs, can be determined by the shape of the isobar. Ridges are usually associated with very little wind.
“If you’re a Bayview Mackinac sailor,” says Thornton, “a ridge across the race course is your worst nightmare.”
A surface weather map also allows you to identify “fronts.” A front (whether it’s a cold front, a warm front, or a stationary front) is an indication of where the boundary lies between air masses of differing temperature and moisture characteristics, such as a cooler air mass next to a warmer air mass.
Interestingly, warm fronts move an average of 10 knots while cold fronts move an average of 20 knots.
“Weather happens where air masses meet,” Thornton warns.
Predicting Wind
Sailors look for the wind and name it by the direction it is coming from. On a weather map, you’ll find “wind barbs”—symbols resembling small flags—indicating wind direction and speed. The staff points in the direction that the wind is coming from, and a combination of half barbs (5 knots) and full barbs (10 knots) show the wind speed. A ‘pennant’ is used when winds reach 50 knots.
Forecast graphics from the National Weather Service’s Great Lakes Portal website show wind direction, wind speed, and other variables at three-hour intervals. Other weather providers, such as PredictWind, offer forecasts at one-hour intervals, which are better for short-term outings and regattas.
Sailors can also estimate wind direction and speed from surface weather maps. This is because wind behaves differently around areas of high and low pressure. Wind flows clockwise and outward from an area of high pressure, and counter-clockwise and inward around an area of low pressure. Relative wind speed can be estimated based on how close the isobars are; the closer that the isobars are to each other, the higher the wind speed. Overall, this type of wind (created by pressure systems) is referred to as “large-scale” wind.
“Small-scale” winds, such as lake and land breezes, often form when the large scale winds are weak. These winds are thermally driven, meaning they are driven by differences in temperature. This is also a complex subject better understood inside a classroom, but at its most simple, the wind blows toward the warmer entity, toward the water or toward the land. This also means that the intensity with which wind blows varies with the time of day and the intensity of the sun’s heat.
When You See Dark Clouds
Thunderstorms can quickly spoil an outing in many ways: strong winds, large waves, dangerous lightning, waterspouts, or visibility-limiting rain. Reading the marine forecast and the Hazardous Weather Outlook from your local National Weather Service office before leaving the dock is a great way to avoid an unexpected encounter with a thunderstorm.
“Creating a thunderstorm is a bit like baking a cake in that you need the ingredients in the right proportion at the same place at the same time,” explains Thorton. The primary ingredients for thunderstorm formation are: relatively warm and moist air near the surface; colder and drier air aloft; an atmosphere that promotes upward motion (instability); and a source of lift to jumpstart the storm’s updraft. In many instances, the source of lift is a cold, warm, or stationary front accompanying a low pressure system.
The temperature of the Great Lakes plays an important role in the evolution of thunderstorms. Early in the season when the lakes are cold, thunderstorms that formed in the warm moist air over land often weaken as they head offshore because the supply of warm moist air (the storm’s energy source) is cut off.
This situation flip-flops in the fall as cooler air flows over the relatively warmer lake. The instability associated with the temperature difference between the lake and the overlying air can intensify thunderstorms as they move offshore. This is also the reason that waterspouts are more common in the late summer and early fall.
Checking Doppler Weather Radar before—and periodically during—your outing is an important part of safe boating. There are many weather apps that make this very easy on an internet connection via your smartphone or a tablet. Thornton has used an app called ‘RadarScope’ for several years and has found it to be a very reliable radar application.
Due to the curvature of the Earth and the nature of radar beams, thunderstorms that are far from a radar station may appear weaker on radar than they actually are.
“When using radar,” Thornton advises, “always use the station closest to the storm you’re concerned about, not the station closest to you.”
Learning some basic forecasting skills is a great way to make your boating safer and more enjoyable. The schedule for in-person and online classes offered by LakeErieWX Marine Weather can be found at www.lakeeriewx.com/Seminars/Seminars.html.
Mark Thornton contributed to this article.
About the Author
Melissa Walsh was raised on the shores of the heart-shaped lake nestled between the St. Clair and Detroit rivers that pumps the lifeblood of water recreation and the “blue economy” in Southeast Michigan. She’s enjoyed learning to navigate the shallow depths and sail the shifty winds of Lake St. Clair. So naturally, as a freelance journalist, Walsh delights in writing about power boating and sailing.
This article first appeared in the Launch Issue (May/Jun) 2020 of Great Lakes Scuttlebutt magazine.
tags: Boating 101, Education, Navigation, safety
